Such differential pressure sensors are counted as belonging to the state of the art and are formed of a semiconductor substrate which in an inner region is thinned out into a membrane. Four measurement resistances are formed in this inner region, which are connected into a measurement bridge so that on deformation of the membrane, as occurs when impinged by pressure, the pressure acting on both sides of the membrane may be determined in the form of a differential pressure whilst making use of the piezoresistive effect. Differential pressure sensors as well as simple pressure sensors with which atmospheric pressure prevails on one side function according to this principle. Such a semiconductor pressure sensor is known for example from DE 197 01 055 A1.
Since the characteristic curve of the pressure sensor changes due to temperature, it is counted as belonging to the state of the art, apart from the actual measurement resistances formed on the membrane, to arrange so-called compensation resistances in the region next to the membrane which are dimensioned such that their piezoresistive coefficients are equal with regard to magnitude. The resistances are connected in series and are arranged such that their changes due to temperature are compensated.
From EP 0 083 496 B1 it is counted as belonging to the state of the art to arrange the measurement resistances and compensation resistances in each case parallel to one another and to connect these into measurement bridges, in order in this manner to compensate measurement errors caused by temperature. Furthermore measurement differences due to the pressure level are here too compensated by the circuit arrangement. However with the arrangement known from EP 0 083 496 B1 the compensation resistances lie outside the membrane region, and specifically where the semiconductor substrate is to be regarded as practically rigid due to a glass tube attached perpendicularly thereto.
The sensor known from DE 197 01 055 A1 as well as from EP 0 083 496 B1 may be applied as a differential pressure sensor, but however in practice is provided for a pressure measurement with respect to the atmospheric pressure acting on the membrane on one side, and thus are not suitable for measuring a differential pressure deviating from this.
For controlling heating circulatory pumps it is counted as belonging to the state of the art to apply a differential pressure senor in the heating circuit in order in this manner to detect the load condition of the heating installation and to control the heating circulatory pump accordingly. At the same time the differential pressure is a measure for the setting of the thermostat valves and thus for the heat requirement of the installation. The system pressure prevailing in the region of the pump differs very considerably depending on the position of installation and the design of the heating, even though the differential pressures lie roughly at the same level.
It is particularly desirable in the previously mention case of application, apart from the differential pressure to detect a system pressure, since this provides information on the extent of filling of the heating installation, so that one may prevent the pump from running dry.